In our last blog on heat treating, lab tests and hardness testing, I mentioned the large number of hardness measuring systems that are in use that then get mathematically converted into the more common systems. You might be wondering why this is done. While there is some proprietary technology in play, not all methods work well on all parts and specifications. Buried in the fine print of operating instructions for the devices are warnings about indenters that can pierce shallow cases and give false readings, or indenters that leave a “dent” on the part surface that is too large for the end user.
Accurate description of the hardness test location is important, as is an understanding of how hardness varies with distance from the surface. Back in the “dark ages” we just looked at the Brinell “dimple” on a chunk of steel and were surprised when it checked “soft” after the shop machined an inch off the outside diameter.
Do not think that “core hardness” means the same thing to everyone. If you need certain mechanical properties in a certain place on the part, you must specify the location on the drawing. As noted above, a single piece of through hardened bar could have several different hardness readings.
This is why critical service parts often require a few “sacrificial lambs” to be processed along with the production parts. The lab has to cut these samples up and check the properties at key locations. You cannot get valid data on round test bars; even slotted or “toothed” coupons have to be engineered to mimic the section properties of the actual components. Many shops just run extra parts or use set-up pieces rejected during machining instead of coupons.
I cannot emphasize enough that if you are not cutting up parts or coupons you do not know what is really happening during your thermal processing. When disputes arise, no one cares what your process control charts or your test bar showed; they will take a failed or rejected part, cut it up, and perform the lab tests you wish you had done originally. The cost will be much higher for “forensic lab work” than if you had done it up front.